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On the science of baguettes, poolish and steam in the oven

Whenever visiting France there is one thing I really really look forward to: visiting a boulangerie, a bakery. The French baguettes are just so good, always fresh (since storing them for even one day will make them go stale) and hard. I also adore the pain au chocolat, a croissant with chocolate filling.

Since we’re not in France that often I decided I had to try making a French baguette myself! Since I didn’t trust any non-French website I decided to google a recipe in French and found one. Why in French? Because I’ve found there are a lot of people calling their white long bread a French baguette whereas it clearly isn’t! Of course, this isn’t the only way to make them, as a French baker will probably be able to tell you.

This French baguette almost tasted like the real thing. It was really crispy on the outside, soft and light on the inside. When making the bread, two special steps seemed to contribute to the result: using a “poolish” & placing a bowl of water in the oven during baking!

Poolish

Never heard of poolish? Neither did I. A poolish a so-called pre-ferment. It is a mixture of flour, water and yeast that is left to proof and leaven for a few hours before starting the bread making process. In the case of poolish you use an equal weight quantity of water and flour, making it quite liquid.

During this leavening time the yeast ferments. It consumes the sugar in the flour and produces gas (carbon dioxide) as well as flavour. The longer a yeast is allowed to ferment, the more flavour molecules it develops. In a poolish the amount of yeast varies. The more yeast you use, the faster it ferments and the faster the poolish is ‘ready’.

Improves extensibility

Since the flour is the poolish has had a long time to hydrate it will become noticeably more flexible as well. This is similar to what you see happening when making a no-knead bread. Even though the dough might look stiff at the start,it becomes noticeably more flexible over time. Part of this is due to the hydration of the flour, but it is likely that enzymes in the flour also cause some changes which help strengthen the gluten network.

Difference with a sourdough starter

To make a poolish you need some yeast. Once the poolish is ready to use, that is it has bubbled up, it needs to be used within a few hours. You cannot store a poolish for days on end. (The best way to extend the shelf life somewhat is storing it in the fridge, but this will change the flavour profile.) A sourdough starter on the other hand uses wild yeasts from the air and can be kept alive for months or years on end with the right feeding regime.

poolish plus dough
On the left a poolish, ready to be mixed with the other ingredients (on the right). The poolish has a lot of bubbles but hasn’t yet collapsed, making it ready for use.

Bowl of water in the oven for crispyness

The second trick I used was one I discovered when writing and reading about the history of ovens. I discovered that at the time, when big stone ovens were used, the moisture content in an oven was very different.

These ovens used to be very large and in the first stages of baking bread (or pizza dough) a lot of moisture would evaporate from the dough. This moisture couldn’t really go anywhere, so would just stay in the oven.

Nowadays, ovens aren’t designed for that anymore, so if you have a conventional oven you’ll have to increase the moisture content yourself. I do that be taking an oven proof glass bowl and filling this with boiling hot water which I then place in the oven. Since my oven heats up pretty quickly and water tends to take a long time to heat up, I prefer adding water that’s already pretty hot.

Once the water is hot part of it will evaporate and form steam. Why is this important? I’ve found several sources (one of which is this article from serious eats) which mention the same two main reasons:

  1. The high moisture content in the oven will keep the outside of the bread flexible for a longer period of time. This should allow it to expand more extensively than without the steam.
  2. Moisture is good in transferring heat and a high moisture content should increase the temperature transfer at the surface (thus crust) of your bread. Apparently, this should lead to more browning but also a thicker crust.

The reasons make sense to me and my experience with the method has indeed been very positive. Never have I had a bread with such a good crunchy, crispy crust as I had this time!

Sullivan bakery's whole wheat bread, left in closed dutch oven, right open tray
No, these are not baguettes but show the effect of using a closed off space filled with moisture tremendously well. Both are the same recipe. However, the left bread has been baked in a closed cast iron pan with a lid on top whereas the right was baked in the same oven, at the same time, but on a regular baking tray. Not only did the crust of the one baked in the closed space look a lot better, it was also a lot crunchier. Baking in a closed off space has a very similar effect as adding a bowl of moisture in the oven, you’re increasing the moisture content around the bread. Only towards the end of baking did I lift off the lid.

Good luck and let me know how it went! Also, if you have any other (bread) baking/cooking questions which are related to science, let me know! Who knows you might be featured in a future post.

French baguette recipe

The recipe I used comes from Recette Dessert, I didn’t make a lot of changes since it turned out great the way it did!

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French baguette – Poolish + Bowl of Water in Oven

  • Author: Science Chef
  • Category: Bread
  • Cuisine: French

Scale

Ingredients

Poolish (this is a sort of starter)

  • 170g flour (regular flour)
  • 170g water
  • 1/2 tsp yeast (I use instant yeast)

Dough

  • The poolish from the previous step
  • 330g flour
  • 180g water
  • 1/4 tsp salt
  • 1 tsp yeast

Instructions

  1. Mix the flour, water and yeast for the poolish. It will look pretty wet and sticky which is fine, you don’t have to knead it, only mix it.
  2. Leave the poolish to stand for approx. 12 hours at room temperature, I made mine in the morning and used it in the evening, it stood for a total of 10 hours.
  3. Add all ingredients for the dough into a stand mixer (or knead by hand) and leave to mix at a low speed for 15 minutes.
  4. Leave the dough to rest for another 45 minutes. Make sure to cover the bowl with dough with plastic wrap to prevent it from drying out.
  5. Split the dough into 3 portions and shape them into little baguettes. Cover the baguettes with a towel and some plastic wrap and leave for another 45-60 minutes.
  6. Turn on the oven (fan) to 230C. Place a bowl with hot boiling water at the bottom of your oven.
  7. Once the oven reached its temperature and once the water is (still) around 100C, put the baguettes in for 30 minutes (the baking time will depend on the shape of your breads, long and thin will bake quicker than short and thick).
  8. Enjoy!

Further reading

Want to read more about poolish? The Weekend Bakery wrote a nice post about its use.

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